The present invention relates to sensors for sensing the presence of a liquid, suitable for use as liquid level sensors in a tank, the sensor being of the type comprising a probe containing a resistor having a negative temperature coefficient (NTC), an electrical power source for dissipating power in the probe by Joule effect, and means for comparing the resistance of the probe with a determined threshold, the resistors being used as thermistors.
Such sensors are based on the difference in thermal conductivity between liquids and gases. For the same heat power of a thermistor, the equilibrium temperature achieved is higher when the thermistor is in a gas than when it is in a liquid. By measuring the voltage difference across the terminals of the thermistor and given the current flowing through it, it is possible to determine its resistance and the medium in which the thermistor is located.
Present sensors operating on this basis suffer from severe limitations. In particular, they operate satisfactorily only when the temperatures of the liquid and of the gas vary little. Otherwise, it is not possible to determine the medium in which the probe is located merely by comparing its resistance with a single determined resistance. FIG. 1 illustrates this limitation for the particular case of a fuel level sensor placed in a tank and surmounted by air, the thermistor being of the NTC (negative temperature coefficient) type having an electrical resistance that varies in a manner that is close to being a negative exponential. The resistance R.sub.1 of the NTC varies responsive to temperature .THETA. as shown by curve 10 when it is immersed in air, and as shown by curve 12 when it is immersed in fuel. If a constant discrimination threshold R.sub.0 is used, then the level can be sensed only between two temperatures T1 and T2 that are relatively close together.
Another prior art sensor (French patent No. 2 515 342) uses a PTC whose resistance increases suddenly at a transition temperature. The total resistance of two thermistors in series is not measured. The thermistors are connected to a conventional power source.